The present invention relates to a belt for bending glass sheets and to a method for bending a glass sheet using the same.
Bent glass sheets are widely used for vehicles and buildings, and especially in the field of window glass for vehicles, there is a great demand for bent glass sheets due to design and aerodynamic considerations. Flat glass sheets that have been mass-produced by the float process are formed into bent glass sheets in a secondary process, for which a number of industrial methods are known. Moreover, if the glass sheets are heated for the bending step, they often are quenched afterwards to temper them.
Press molds are used in most of the methods for bending glass sheets that have been suggested so far. However, during the forming step in these methods for bending glass sheets by press forming, the glass sheets have to be temporarily stopped in the press mold.
With regard to the manufacturing efficiency of continuous production of bent glass sheets, methods have been proposed wherein the glass sheets are bent without stopping them on the manufacturing line. In some of these methods, the glass sheets are heated and softened inside a furnace while conveying them horizontally, and each of the glass sheets is gradually adapted to the surface form of a glass conveying means, for example an airbed, using the glass sheet""s own weight (sag bending). These methods are very efficient for continuous production of glass sheets having the same curved surface shape, and have been employed with various improvements (see for example Publication of Unexamined Japanese Patent Application No. Hei 7-237928).
Moreover, Publication of Unexamined Japanese Patent Application No. Sho 55-75930 discloses a method wherein a glass conveying passage having a curvature is set up in continuation to the carry-out opening of the furnace, and the glass sheets are bent while being conveyed by rollers along this conveyer passage. Compared to the method using the glass sheet""s own weight, this method has a better heating efficiency and the shape of the glass sheets can be exchanged easier.
Other methods have been proposed, wherein, while conveying the glass sheets with rollers, the glass sheets are bent not only in the direction in which they are conveyed, but also in the direction that is perpendicular to the conveying direction (referred to as xe2x80x9ccross directionxe2x80x9d in the following).
For example, Publication of Unexamined Japanese Patent Application No. Hei 3-174334 proposes a method, wherein rollers made of an elastic body are arranged above and below the glass sheets, and by applying a stress onto these roller pairs from the outside, the glass sheets are bent also in the cross direction. Roller pairs for bending glass sheets in a cross direction or for conveying glass sheets that have been bent in the cross direction are disclosed for example in Publication of Unexamined Japanese Patent Application No. Sho 54-85217 and No. Sho 55-75930.
However, in methods wherein the glass sheets are bent by rollers that are arranged above and below the conveyer passage, the surfaces of the glass sheets are pressed locally. Consequently, there is the problem that roller marks can appear easily on the surfaces of the glass sheets. Moreover, in eel intermittent bending with rollers, there is the problem that the freedom and precision with which the glass sheets can be formed are not sufficient. When bending with rollers, the glass sheets are bent while being suspended between the rollers. Therefore, it is difficult to attain the desired curved shape at the front end and the rear end in the conveying direction of the glass sheet. Moreover, when bending the glass sheets with rollers that are arched by elastic deformation, it is difficult to obtain precisely the desired curved shape.
Methods have been proposed, wherein the glass sheets are bent while contacting not rollers but a belt. For example, Publication of Unexamined Japanese Patent Application No. Hei 3-50132 discloses a method wherein a glass sheet is carried out of a furnace, conveyed by a belt to a pressing position, and press-formed together with the belt. This method takes advantage of the belt""s flexibility to reduce the glass sheet""s temperature decrease between heating and quenching. That is, the glass sheet is bent while it contacts the belt, and it is also quenched in this situation. Furthermore, Publication of Unexamined Japanese Patent Application No. Hei 2-221133 and No. Hei 6-40732 propose methods for bending and tempering glass sheets that are improvements of this method.
However, in the methods for bending the glass sheets by pressing them together with a belt, each of the glass sheets has to be temporarily stopped, so that there is still room for improvement regarding the manufacturing efficiency. Moreover, Publication of Unexamined Japanese Patent Application No. 3-50132 discusses the thermal conductivity of the belt in detail, but makes no mention of the defects in the surface of the glass sheet that occur due to the contact with the belt.
Thus, regarding these methods for bending a glass sheet while contacting it with a member using a glass conveyer means such as a rollers or a belt, there has still not been enough research concerning the defects on the surface of the glass sheet that occur due to the contact with the member, although these defects have a considerable influence on the final product quality of the bent glass sheet.
It is an object of the present invention, which has been conceived upon consideration of these circumstances, to provide a belt for bending a glass sheet that suppresses defects in the surface of the bent glass sheet. It is another object of the present invention to provide a method for bending a glass sheet with high manufacturing efficiency using this belt.
In order to achieve these objects, a first belt for bending a heated glass sheet having a heat-resistant surface for pressing a surface of the glass sheet comprises a strip of a heat-resistant member for forming the heat-resistant surface. The heat-resistant member is arranged so that the heat-resistant surface defines an endless track and end surfaces of the heat-resistant member are not exposed at the heat-resistant surface.
It is preferable that in this belt, (a) two end surfaces of the heat-resistant member, (b) two bent portions formed by folding the two end surfaces from an outer periphery of the endless track to an inner periphery of the endless track, or (c) one end surface of the heat-resistant member and one bent portion formed by folding the other end surface from an outer periphery of the endless track to an inner periphery of the endless track oppose each other on the endless track.
In this technical field, conventional belts are formed into loop-shape by simply overlapping the belt ends and joining the ends together. When the bending is performed using a belt instead of rollers, defects on the surfaces of the glass sheets can be reduced. However, there was the possibility that the step portion formed by the local overlap of the belt ends caused a defect in the surfaces of the glass sheets. In contrast, the inventive belt eliminates the local pressure difference applied to the surface of the glass sheet by eliminating this step portion.
A second belt for bending a heated glass sheet having a heat-resistant surface for pressing a surface of the glass sheet also comprises a strip of a heat-resistant member for forming the heat-resistant surface and the heat-resistant member is arranged so that the heat-resistant surface defines an endless track. This heat-resistant member has fitting portions arranged along a pair of side-edges of the endless track.
In this technical field, conventional belts are used while applying a tensional force only in the conveying direction of the glass sheet. However, these belts may be deformed or become wavy during the bending of the glass sheet, which causes defects in the surface of the glass sheet. In contrast, a tensional force is applied in the cross direction of the inventive belt, so that the surface of the belt is stabilized.
A method for bending a glass sheet in accordance with the present invention comprises bending a heated glass sheet by pressing the heat-resistant surface of the inventive belt against the glass sheet, while rotating the belt along the endless track and conveying the glass sheet along a portion of the endless track. With this method, bent glass sheets with suppressed surface defects can be manufactured with high efficiency.
It is preferable that the glass sheet is bent at least in a direction that is perpendicular to said endless track. If a belt having fitting portions is used, it is preferable that the belt is rotated using rollers that have members that fit into the fitting portions.